Switchable light modulator device comprising polymer structures that create a plurality of cavities that are sealed with a fluid comprising electrophoretic particles

ABSTRACT

A switchable light modulator device (201, 202, 203, 204, 205) comprises a first substrate (101, 102, 103) and a second substrate (141, 142, 143, 144) with opposite major surfaces spaced apart by one or more polymer structures that each comprise two or more parts and define wall features (21b, 22b, 23b) for a plurality of cavities (111, 112, 113, 114), the cavities sealing a fluid (71, 72, 73, 74) or gel in discrete volumes. Each of the one or more polymer structures comprises a mould part (21, 22, 23) bonded to the first substrate and defining a recess (31, 32, 33), and a cast part (81, 82, 83, 84) filling the recess and bonded to the second substrate and a surface of the recess, the cast part being defined by the surface of the recess and the second substrate replicating the surfaces of both.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/582,042, filed Jan. 24, 2022, which is itself a continuation of U.S.patent application Ser. No. 17/193,051, filed Mar. 5, 2021, now U.S.Pat. No. 11,237,419, which claimed the benefit of and priority to GreatBritain Patent Application Serial No. 2003224.9 filed on Mar. 5, 2020,the contents of which are incorporated by reference herein in itsentirety.

FIELD OF THE INVENTION

The present invention relates to light modulator devices that areoptically switchable and comprise a fluid or gel layer, and especiallyto devices that have flexible substrates. Such devices preferably have apolymer structure within the viewing area to maintain the gap betweenthe substrates and to support handling in use including bending andlaminating to glass or another substrate. Examples of productapplications include switchable smart windows, outdoor informationdisplays, and flexible display devices.

BACKGROUND OF THE INVENTION

This invention relates to light modulators, that is to say to variabletransmission windows, mirrors and similar devices designed to modulatethe amount of light or other electro-magnetic radiation passing therethrough. For convenience, the term “light” will normally be used herein,but this term should be understood in a broad sense to includeelectro-magnetic radiation at non-visible wavelengths. For example, asthe present invention may be applied to provide windows which canmodulate infra-red radiation for controlling temperatures withinbuildings. More specifically, this invention relates to light modulatorswhich use particle-based electrophoretic media to control lightmodulation. Examples of electrophoretic media that may be incorporatedinto various embodiments of the present invention include, for example,the electrophoretic media described in U.S. Pat. No. 10,809,590 and U.S.Patent Publication No. 2018/0366069, the contents of both of which areincorporated by reference herein in their entireties.

In the prior art, solutions that have polymer structure in the fluid orgel layer, and suitable for use with the invention include U.S. Pat. No.8,508,695 to Vlyte Innovations Ltd., which discloses dispersing fluiddroplets (1 to 5 microns in diameter) in a continuous polymer matrixthat is cured in place to both substrates, to contain liquid crystals.Additionally, U.S. Pat. No. 10,809,590 to E Ink Corporation disclosesmicroencapsulating fluid droplets and deforming them to form a monolayerof close packed polymer shells in a polymer matrix on one substrate andsubsequently applying an adhesive layer to bond the capsule layer to asubstrate. Also EP1264210 to E Ink California discloses embossing amicro-cup structure on one substrate, filling the cups with fluid havingpolymerizable components and polymerizing the components to form asealing layer on the fluid/cup surface, then applying an adhesive layerto bond to the second substrate. Additionally, EP2976676 to VlyteInnovations Ltd. discloses forming a wall structure on one substrate,coating the tops of walls with adhesive, filling the cavities defined bythe walls with fluid, and polymerizing the adhesive to bond the tops ofwalls to the opposing substrate.

Many of these prior art solutions impose limitations in order to providea workable solution for isolating one specific fluid (e.g., liquidcrystal (LC)) for one specific application (e.g. switchable LC film). Inorder to do this, all of the above solutions expose the electro-opticalfluid to prepolymer components and a polymerization step. This forcescompromises and adds complexity. For example, the electro-optical fluidcomponents must not participate in or hinder the polymerization and theprepolymer components must phase separate from the fluid onpolymerization and somehow form solid polymer structure in defined areas(e.g. only on the fluid surface of a micro-cup). In addition, it can bedifficult to develop strong chemical bonds to the surface of substratesin the presence of a fluid because the fluid can preferentially wet thesurface undermining peel adhesion. Furthermore, there will be residualcomponents, including unused monomer, low molecular weight polymer, andnanoparticles from a polymerization step conducted in contact with thefluid that can contaminate or otherwise compromise switching of theelectro-optical fluid. All of these conditions can lead to failure ofthe end product, because of lack of optical activity, delamination, orleakage of the internal fluid.

In EP3281055, to Vlyte Innovations Ltd., the electro-optical fluid isnot exposed to a polymerization step. EP3281055 describes a flexibledevice including solid polymer microstructures embedded in its viewingarea and the microstructures are on both substrates. The microstructuresjoin (i.e. fasten) the substrates of the device to each other byengaging with each other over a length orthogonal to the substrates. Thejoined microstructures incorporate a wall structure that divides adevice's fluid layer into a monolayer of discrete volumes containedwithin corresponding cavities. This provides the device with significantstructural strength. In the method described, mating microstructures(i.e. male and female parts) are formed on each substrate, thenprecisely aligned with each other and joined in a press fit that alsoseals the fluid layer in the cavities. As noted earlier, theelectro-optical fluid is not exposed to a polymerization step. Alimitation of the method is that it requires precise alignment anddimensional stability in the X and Y axis of the faces to be joined overlarge distances, typically over one or more meters in smart glassapplications.

Particle-based electrophoretic displays, in which a plurality of chargedparticles move through a suspending fluid under the influence of anelectric field, have been the subject of intense research anddevelopment for a number of years. Such displays can have attributes ofgood brightness and contrast, wide viewing angles, state bistability,and low power consumption when compared with liquid crystal displays.The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin published U.S. patent application Ser. No. 2002/0180687 that someparticle-based electrophoretic displays capable of gray scale are stablenot only in their extreme black and white states but also in theirintermediate gray states, and the same is true of some other types ofelectro-optic displays. This type of display is properly called“multi-stable” rather than bistable, although for convenience the term“bistable” may be used herein to cover both bistable and multi-stabledisplays.

As noted above, electrophoretic media require the presence of asuspending fluid. In most prior art electrophoretic media, thissuspending fluid is a liquid, but electrophoretic media can be producedusing gaseous suspending fluids; see, for example, Kitamura, T., et al.,“Electrical toner movement for electronic paper-like display”, IDWJapan, 2001, Paper HCS1-1, and Yamaguchi, Y, et al., “Toner displayusing insulative particles charged triboelectrically”, IDW Japan, 2001,Paper AMD4-4). See also European Patent Applications 1,429,178;1,462,847; and 1,482,354; and International Applications WO 2004/090626;WO 2004/079442; WO 2004/077140; WO 2004/059379; WO 2004/055586; WO2004/008239; WO 2004/006006; WO 2004/001498; WO 03/091799; and WO03/088495. Such gas-based electrophoretic media appear to be susceptibleto the same types of problems due to particle settling as liquid-basedelectrophoretic media, when the media are used in an orientation whichpermits such settling, for example in a sign where the medium isdisposed in a vertical plane. Indeed, particle settling appears to be amore serious problem in gas-based electrophoretic media than inliquid-based ones, since the lower viscosity of gaseous suspendingfluids as compared with liquid ones allows more rapid settling of theelectrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT), E Ink Corporation, E InkCalifornia, LLC and related companies describe various technologies usedin encapsulated and microcell electrophoretic and other electro-opticmedia. Encapsulated electrophoretic media comprise numerous smallcapsules, each of which itself comprises an internal phase containingelectrophoretically-mobile particles in a fluid medium, and a capsulewall surrounding the internal phase. Typically, the capsules arethemselves held within a polymeric binder to form a coherent layerpositioned between two electrodes. In a microcell electrophoreticdisplay, the charged particles and the fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. Thetechnologies described in these patents and applications include:

-   (a) Electrophoretic particles, fluids and fluid additives; see for    example U.S. Pat. Nos. 7,002,728 and 7,679,814;-   (b) Capsules, binders and encapsulation processes; see for example    U.S. Pat. Nos. 6,922,276 and 7,411,719;-   (c) Microcell structures, wall materials, and methods of forming    microcells; see for example U.S. Pat. Nos. 7,072,095 and 9,279,906;-   (d) Methods for filling and sealing microcells; see for example U.S.    Pat. Nos. 7,144,942 and 7,715,088;-   (e) Films and sub-assemblies containing electro-optic materials; see    for example U.S. Pat. Nos. 6,982,178 and 7,839,564;-   (f) Backplanes, adhesive layers and other auxiliary layers and    methods used in displays; see for example U.S. Pat. Nos. 7,116,318    and 7,535,624;-   (g) Color formation and color adjustment; see for example U.S. Pat.    Nos. 7,075,502 and 7,839,564;-   (h) Methods for driving displays; see for example U.S. Pat. Nos.    7,012,600 and 7,453,445;-   (i) Applications of displays; see for example U.S. Pat. Nos.    7,312,784 and 8,009,348; and-   (j) Non-electrophoretic displays, as described in U.S. Pat. No.    6,241,921 and U.S. Patent Applications Publication No. 2015/0277160;    and applications of encapsulation and microcell technology other    than displays; see for example U.S. Patent Application Publications    Nos. 2015/0005720 and 2016/0012710.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned 2002/0131147. Accordingly, for purposes of thepresent application, such polymer-dispersed electrophoretic media areregarded as sub-species of encapsulated electrophoretic media.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the suspending fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, International Application Publication No. WO 02/01281, andpublished U.S. application Ser. No. 2002/0075556, both assigned to SiPixImaging, Inc.

Electrophoretic media are often opaque (since, for example, in manyelectrophoretic media, the particles substantially block transmission ofvisible light through the display) and operate in a reflective mode.However, electrophoretic devices can also be made to operate in aso-called “shutter mode,” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, theaforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat.Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856.Dielectrophoretic displays, which are similar to electrophoreticdisplays but rely upon variations in electric field strength, canoperate in a similar mode; see U.S. Pat. No. 4,418,346. Other types ofelectro-optic displays may also be capable of operating in shutter mode.In particular, when this “shutter mode” electrophoretic device isconstructed on a transparent substrate, it is possible to regulatetransmission of light through the device.

An encapsulated or microcell electrophoretic display typically does notsuffer from the clustering and settling failure mode of traditionalelectrophoretic devices and provides further advantages, such as theability to print or coat the display on a wide variety of flexible andrigid substrates. (Use of the word “printing” is intended to include allforms of printing and coating, including, but without limitation:pre-metered coatings such as patch die coating, slot or extrusioncoating, slide or cascade coating, curtain coating; roll coating such asknife over roll coating, forward and reverse roll coating; gravurecoating; dip coating; spray coating; meniscus coating; spin coating;brush coating; air knife coating; silk screen printing processes;electrostatic printing processes; thermal printing processes; ink jetprinting processes; electrophoretic deposition; and other similartechniques.) Thus, the resulting display can be flexible. Further,because the display medium can be printed (using a variety of methods),the display itself can be made inexpensively.

One potentially important market for electrophoretic media is windowswith variable light transmission. As the energy performance of buildingsand vehicles becomes increasingly important, electrophoretic media couldbe used as coatings on windows to enable the proportion of incidentradiation transmitted through the windows to be electronicallycontrolled by varying the optical state of the electrophoretic media.Effective implementation of such “variable-transmissivity” (“VT”)technology in buildings is expected to provide (1) reduction of unwantedheating effects during hot weather, thus reducing the amount of energyneeded for cooling, the size of air conditioning plants, and peakelectricity demand; (2) increased use of natural daylight, thus reducingenergy used for lighting and peak electricity demand; and (3) increasedoccupant comfort by increasing both thermal and visual comfort. Evengreater benefits would be expected to accrue in an automobile, where theratio of glazed surface to enclosed volume is significantly larger thanin a typical building. Specifically, effective implementation of VTtechnology in automobiles is expected to provide not only theaforementioned benefits but also (1) increased motoring safety, (2)reduced glare, (3) enhanced mirror performance (by using anelectro-optic coating on the mirror), and (4) increased ability to useheads-up displays. Other potential applications of VT technology includeprivacy glass and glare-guards in electronic devices.

SUMMARY OF THE INVENTION

Described herein is an improved architecture for a switchable lightmodulator that can be used for a window, mirror, display, sun shade, orsign, among many other applications. In particular, the described designis more robust than variable transmission devices such as electrochromicfilms, and provides a better viewing experience due to an improved clear(open state) with reduced haze.

In a first aspect, the switchable light modulator includes a firstsubstrate having a first major surface, a second substrate having asecond major surface, a polymer wall structure having a top and abottom. The polymer wall structure is disposed between the first majorsurface and the second major surface, thereby creating a plurality ofcavities that contain a modulating fluid or a modulating gel in discretevolumes within the cavities. The polymer wall structure includes a mouldpart defining a recess along the top of the polymer wall structure, anda cast part formed by disposing a fluid pre-cursor into the recess andfilling the recess, and subsequently curing the fluid pre-cursor to bondthe second substrate to a surface of the recess. In one embodiment, thebottom of the polymer wall structure is bonded to the first substrate.In one embodiment, the fluid pre-cursor does not contact the modulatingfluid or the modulating gel. In one embodiment, the fluid pre-cursorextends beyond side walls defining the recess, and into the cavity. Inone embodiment, the mould part is optically transparent and the castpart obscures light and includes a colorant, a filler material, or alight scattering material. In one embodiment, the mould includes acolorant, and the colorant matches a colour of a particle that isdisposed in the modulating fluid or modulating gel. In one embodiment,the fluid pre-cursor comprises an elastomeric polymer having a glasstransition temperature (Tg) less than 20° C. In one embodiment, theelastomer polymer is a polyurethane. In one embodiment, the recess has amaximum depth that is greater than or equal to 5% of the orthogonaldistance between the first major surface and the second major surface.In one embodiment, the cavities are between 0.3 mm and 3 cm in longestdimension, and the center-to-center distance of adjacent cavities isbetween 0.6 mm and 10 cm. In one embodiment, mould parts havedifferences in the respective shapes of their recesses includingvariation in the depth and width of the recesses. In one embodiment, thepolymer wall structures additionally include bracing features. In oneembodiment, the first substrate or the second substrate comprises aflexible transparent material. In one embodiment, the switchable lightmodulator has a first state that strongly attenuates light, and a secondstate that is substantially transparent to visible light. In oneembodiment, the modulating fluid or the modulating gel includeselectrophoretic particles, liquid crystals, a combination of polar andnon-polar liquids, an electrochromic fluid, a thermochromic fluid, or aphotochromic fluid.

In another aspect a method of making a switchable light modulator. Themethod includes providing a first substrate including a first majorsurface, providing a second substrate including a second major surface,providing a polymer wall structure having a top and a bottom, whereinthe polymer wall structure includes a mould part that defines a recessalong the top of the polymer wall structure, filling the recess with afluid pre-cursor, providing a modulating fluid or modulating gel indiscrete volumes within said plurality of cavities, disposing thepolymer wall structure between the first major surface and the secondmajor surface, and curing the fluid pre-cursor to bond the secondsubstrate and a surface of the recess together. In one embodiment, thewall structure is bonded to the first substrate before the step ofproviding a modulating fluid or modulating gel in discrete volumeswithin said plurality of cavities. In one embodiment, curing the fluidpre-cursor to bond the second substrate and a surface of the recesstogether comprises heating the fluid pre-cursor or exposing the fluidprecursor to UV light. In one embodiment, disposing the polymer wallstructure between the first major surface and the second major surfacefurther includes compressing the polymer wall structure between thefirst and second substrates with a roller.

In another aspect, a switchable light modulator device having a firstsubstrate and a second substrate with opposite major surfaces spacedapart by one or more polymer structures that each comprise two or moreparts and define wall features for a plurality of cavities, saidcavities sealing a fluid or gel in discrete volumes, wherein each ofsaid one or more polymer structures comprises a mould part bonded tosaid first substrate and defining a recess, and its cast part fillingsaid recess and bonded to said second substrate and a surface of saidrecess, said cast part being enclosed by said surface of said recess andsaid second substrate replicating the surfaces of both.

In a further aspect, there is provided a switchable light modulatordevice. In some embodiments, the mould part is optically transparent(i.e. comprising only optically transparent polymer) and the cast partobscures light. Light is obscured by the cast part by dispersing orsolubilizing in its polymer structure one or more of: a colorant, afiller material, or a light scattering material. Preferably, the colorof the colorant is selected to match the color or tint of one or moreswitchable light states of such embodiments.

A particular advantage of keeping the mould part optically transparentis that when it is formed by an embossing process that relies on rapidultra-violet (UV) initiated polymerization then absorption of the UV isminimized By contrast, if the mould has light absorbing material thenpolymerization of deep wall sections (e.g., 20 microns or more) would atleast be slowed and most likely would not be possible. In a roll-to-rollprocess equipped with an embossing drum the mould precursor will haveseconds to cure before releasing/peeling from the drum surface. Withsuch a manufacturing process for the mould part it is important to usean optically transparent precursor. Advantageously in embodiments thecast part is cast in place in the device and so cast parts with lightabsorbing material can be thermally cured over a suitably long timeperiod.

These and other aspects of the present invention will be apparent inview of the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example,with reference to the accompanying three-dimensional drawings, in which:

FIG. 1A shows first substrate 101 including its mould microstructure 21and recess 31.

FIG. 1B shows first substrate 101 with recess 31 filled by prepolymer41.

FIG. 1C shows embodiment 201 including mould microstructure 21 and castmicrostructure 81.

FIG. 2A shows first substrate 102 including its mould microstructure 22and recess 32.

FIG. 2B shows embodiment 202 including mould microstructure 22 and castmicrostructure 82.

FIG. 3A shows first substrate 103 including its mould microstructure 23and recess 33.

FIG. 3B shows embodiment 203 including mould microstructure 23 and castmicrostructure 83.

FIG. 4 shows embodiment 204 including mould microstructure 21, castmicrostructure 84, and thin second substrate 144.

FIG. 5 shows embodiment 205 comprising embodiment 204 fixed to an activematrix backplane 165.

The drawing depict one or more implementations in accord with thepresent concepts, by way of example only, not by way of limitations.

DETAILED DESCRIPTION

Embodiments of the present invention provide a switchable lightmodulator device with a fluid layer. The device has solid polymerstructures embedded in its fluid layer and the structures have a height(orthogonal to the juxtaposed major faces of the substrates) and widthon the micron scale. The polymer structures are referred to asmicrostructures (or micro-structures) in this document. The polymermicrostructures are arranged in two parts with the first part bonded tothe first substrate and the second part bonded to the second substrate.The two parts are also bonded to each other and as a consequence join orfix the substrates of the device to each other. The first partincorporates a wall feature that divides the device's fluid layer into amonolayer of discrete volumes corresponding to cavities, and the secondpart incorporates a sealant feature that seals the volumes so thatcavities are isolated from each other. Even though the second partseals, its polymer structure is near completely isolated from contactwith the device's fluid layer by the polymer of the first part.

In embodiments the first part of the two-part polymer structures arereferred to as mould microstructures and they are covalently bonded tothe inner major face of the first substrate. The mould microstructuresare made by micro-replicating the surface of a tool onto the firstsubstrate in an embossing or moulding step. The mould microstructuresare patterned with recess features (the recess feature could also becalled a channel, a notch, or an indentation). In embodiments therecesses are filled by the second part of the two-part polymerstructures. This second part is referred to as cast microstructures andthey are covalently bonded to the inner face of the second substrate.The cast microstructures replicate the recesses by being cast from thembut are not separated from the mould microstructures after casting.

The prepolymer used for casting is printed or otherwise coated to fillthe recesses in the mould microstructures. Then the device is assembledwith its fluid layer disposed between the opposite spaced apart majorsurfaces of the first and second substrates. The mould microstructuresextend from the major surface of the first substrate into the fluidlayer and contact the opposite major face of the second substrate. Inthis way, the mould microstructures define the cell gap of the fluidlayer. At this stage, the recesses are filled with the castmicrostructures' prepolymer and the cavities are filled with the fluid.Next, the prepolymer is polymerized in the casting step to form the castmicrostructures and covalently bond them to the mould microstructuresand to the inner major face of the second substrate. Consequently, thecasting step succeeds the moulding step and occurs in place with thefluid layer and between the substrates. During polymerization (i.e.,casting) the prepolymer bulk in the recess has no contact with the fluidand after polymerization the cast is enclosed by the mouldmicrostructure and isolated from the fluid. Embodiments arecharacterised by two-part polymer structures comprising a mouldmicrostructure part and its cast microstructure part.

The light modulator of embodiments selectively changes one or more oflight attenuation, colour, specular transmittance, or diffuse reflectionin response to electrical, optical or thermal changes and switchesproviding two or more light states. Preferably, light states include oneextreme state that is transparent to visible light and another thatstrongly attenuates light. An important application for embodiments isin smart windows. Some embodiments incorporate the device into a windowas a layer within a glass laminate. In other embodiments, the device isflexible and bonds to a glass pane. In both smart window embodiments,the film device has significant structural strength andcompartmentalises the fluid layer with each discrete fluid volumeself-sealed. The structural strength of embodiments derives from thedesign of its mould and cast microstructures and selection of theirpolymer materials. The structural strength includes that necessary towithstand the glass lamination or bonding process, to withstand theloads encountered when handling and installing large smart windows, andto withstand loads placed on the device over its life by environmentalshocks such as wind and temperature extremes. Furthermore, in transportapplications the device's polymer structure is selected to be resistantto vibrations.

Other embodiments for the device include use as a light shutter, a lightattenuator, a variable light transmittance sheet, a variable lightabsorptance sheet, a variable light reflectance sheet, a mirror, a sunvisor for a vehicle, an electronic skin, a monochrome display, a colourdisplay, or a see-through display. Advantageously, embodiments areparticularly suited to applications that require a large area such asfrom 0.25M2 to 5M2. Furthermore, a device that is a roll of film canhave an area of 1,000M2 or more.

Embodiments are described with reference to the three dimensionalprojection views shown in the figures. FIGS. 1 a to 1 c are used todescribe embodiment 201. FIGS. 2A and 2B describe embodiment 202, FIGS.3A and 3B describe embodiment 203, FIG. 4 describes embodiment 204, andFIG. 5 describes embodiment 205. In the figures the embodiments comprisea fluid or gel layer (71, 72, 73, 74) held between first (101, 102, 103)and second (141, 142, 143, 143) substrates. In some embodiments thefluid layer (71, 72, 73, 74) can be described as an electro-opticallayer, e.g., as described above.

The substrates are spaced apart by polymer micro-structures (21, 22, 23)to define a cell gap (121, 122, 123, 124) for the fluid layer. Themicro-structures also divide the fluid layer into discrete, sealedcavities (111, 112, 113, 114) or compartments. The micro-structures arein two parts, one part is a mould micro-structure (21, 22, 23) thatreplicates the surface of a tool and was formed in an embossing ormoulding step on the first substrate prior to assembling the device, andthe other is a cast micro-structure (81, 82, 83, 84) formed in a recess(31, 32, 33) in the mould micro-structure and on the second substrateafter assembling the device. Consequently, a cast microstructure derivesdirectly from its interface (or intimate contact, or shared surface)with the recess in the mould's microstructure and its interface (orintimate contact, or shared surface) with the second substrate.

In some embodiments one or both substrates is a transparent flexiblefilm (90) that is coated on the fluid side with a transparent electrode(60). The electrodes' major surfaces face each other and are juxtaposedparallel. The opposite surfaces of the substrates form the viewing facesof the embodiments. In alternative embodiments including photochromic orthermochromic light modulators, the substrates (and device) does nothave electrode coatings in the viewing (or switchable area).

In this document the mould microstructures (21, 22, 23) have featuresdescribed as (or corresponding to) cavity walls (21 a, 22 a, 23 a),recesses (31, 32, 33), recess walls (31 a, 32 a, 33 a), and wall braces(23 c). These features, while considered separately from the whole (21,22, 23) of which they are a part, are formed in a single embossing step.By contrast, the cast microstructures (81, 82, 83, 84) are formed in aseparate step and in some embodiments have a different material to themould microstructures.

In FIGS. 1C, 2B, 3B, 4 and 5 just seven complete fluid cavities (111,112, 113, 114) are shown with the section cutting through adjacentcavities and the fluid (71, 72, 73, 74) within these cavities. Thesection through fluid is not shown with hatched lines but the presenceof fluid is generally indicated. The figures of embodiments correspondto a local area (or section) of a much larger device and the figures arenot to scale. In embodiments the pitch of the cavities (or the fluidvolumes defined thereby) is from 50 microns to 3,000 microns. Thelongest dimension (L.D. in FIG. 1A) for a hexagonal cavity, e.g., asshown in FIG. 1A, may be between 0.3 mm and 3 cm. The correspondingcenter-to-center distance (C. to C. in FIG. 1A) may be between 0.6 mmand 10 cm. The relationship between the longest dimension of a cavityand the center-to-center distance may vary depending upon the geometryof the cavities with respect to each other. In some instances, thecavities may be a collection of irregular polygons, which may reduce themoire or other optical interference effect. In one embodiment, a smartglass device with a pitch of 250 microns would typically have between2,000 and 6,000 discrete fluid cavities across its face and from 2,000to 20,000 along its face, or a total number of cavities of between 4million and 120 million. In other embodiments, a larger pitch may beused to improve the viewing experience, i.e., with reduced haze andmoire. When a larger pitch is use, the eye resolves the visible patternas a grid (or array) and perceives the cast parts as a grid of opaqueareas that are the color of the light attenuating particles. In manycases the cast parts are indistinguishable on the face of the lightmodulator when the light modulator is switched to the first(opaque/darkened) light state. When the light modulator is switched toan open, light-transmissive (second) state, the colored particlescollect adjacent the viewable cast parts, effecting an overallappearance of an insect screen. However, the larger size of the cavitiesgreatly improves the haze. Anecdotal studies suggest that for largerapplications (e.g., windows of vehicles or buildings) the presence ofthe viewable cell walls is less objectionable than the higher haze thatmay be present with smaller pitch designs.

FIG. 1A shows first substrate 101 of embodiment 201 (the latter is shownin FIG. 1C). The mould microstructure 21 is a wall structure thatdefines hexagonally shaped cavities 111 and is bonded to the inner faceof substrate 101. The latter is shown in FIG. 1A as microstructure 21bonded to the surface of optional electrode 60 on flexible film 90. Thecavity wall feature (or constituent) of microstructure 21 is indicatedby 21 b and its height orthogonal to the substrate face is shown bydimension 1021 and its width by 1051. On the side opposite to substrate101 the microstructure 21 has a recess 31. The walls (or sides) ofmicrostructure 21 that define recess 31 are indicated by 21 a and shownin magnified view 1001. The width of recess 31 is indicated by 1041 andits height by 1031.

The width 1051 of the majority of wall sections embedded in the viewingarea is from 7.5 microns to 175 microns, more preferably, from 12microns to 125 microns, and most preferably, from 15 microns to 90microns. The width 1041 of the majority of recesses is from 2.5 micronsto 100 microns, more preferably, from 7.5 microns to 85 microns, andmost preferably, from 10 microns to 75 microns. The height (or depth)1031 orthogonal to the second substrate's face for the majority ofrecesses is from 2.5% to 99% of the cell gap 121, more preferable, from5% to 66%, and most preferably, from 6.25% to 46%.

FIG. 1B shows first substrate 101 after prepolymer 41 was printed orcoated into recess 31 in the mould microstructure 21. Examples ofsuitable printing processes for this step in the device assembly (orpreparation or manufacture) include screen printing or ink-jet printing.The preferred printing direction is indicated by arrow 1010. This avoidsprinting into a recess area that is parallel to the printing squeegee.Prepolymer 41 is the precursor to cast 81. Preferably it is a highviscosity (1,000 cst or more) resin curable by free-radicalpolymerization (suitable materials are described later). In FIG. 1B thetop surface of prepolymer 41 is indicated by 41 a. This surfacepreferably coincides or exceeds the wall surface 21 a. In someembodiments an excess of prepolymer 41 can coat the top wall surface 21a after the printing step and before the fluid lamination step.

In FIG. 1C embodiment 201's fluid layer is indicated as 71 and itoccupies part of the volume defined between the optional electrodes 60of substrates 101 and 141. The cell gap 121 corresponds to theorthogonal distance between the respective interfaces of fluid layer 71with the first and second substrates. Fluid 71 is divided into discretefluid volumes by the walls 21 b that in turn are part of mouldmicrostructure 21 with each fluid volume being defined by a cavity 111.The fluid cavities 111 are side-by-side in a hexagonal grid and are in amonolayer. In some embodiments the cavities have an irregular shaperesulting in a side-by-side arrangement having a degree of irregularityor randomness.

Embodiment 201 is assembled in a laminating step using a pair of NIProllers orientated horizontally and having a vertical direction of feed(w.r.t. passage between the NIP rollers). In some embodiments duringlamination the substrates are held under tension by unwinder and/orrewinder stations or modules as part of a roll-to-roll system. Thedevice's fluid 71 is introduced between the substrates 101 and 141forming a reservoir before passing in a vertical orientation through theNIP rollers. The preferred direction of lamination with respect to theorientation of the hexagonal cavities is indicated by arrow 1010 in FIG.1B (described earlier in relation to printing prepolymer 41). In thisorientation fluid 71 does not experience cavity walls that are parallelto the NIP point of the laminating rollers (i.e. parallel to therollers) making it easier to force excess fluid from a cavity as thedevice passes the NIP point. The prepolymer 41 in recess 31 (in themould microstructure 21) is cast in a curing stage to derive castmicrostructure 81 from microstructure 21. Preferably curing is byfree-radical polymerization. The latter is preferably accomplished in ahigh-intensity, ultra-violet radiation module as part of theroll-to-roll process. Alternative curing methods include thermal curing,and alternative types of chain-growth polymerization, include anionic,cationic and coordination polymerization.

Once curing is complete the cast microstructure 81 is strongly bonded tothe second substrate 141 and to the mould microstructure 21. Because thecast microstructure 81 is cast in the volume between the mould's recess31 and the inner face of the second substrate 141, it is surrounded andenclosed by both, and derived and defined by both. The cast 81replicates both interface surfaces and is a 3D imprint of their surfacesand the volume there between. By selecting the prepolymer 41 to bechemically compatible with both surfaces, the cured cast 81 stronglybonds to both. Cast 81 is the child of parent mould microstructure 21,and the two-parts (or pair) are described as mould 21 and its cast 81.

In another embodiment a thermoplastic polymer is applied in liquid formas prepolymer 41 and allowed solidify before the fluid lamination step.After laminating the fluid 71 between the substrates, the casting stepis completed by subjecting the device to high temperature sufficient tocause the thermoplastic polymer 41 to reflow. As it cools thermoplasticpolymer 41 bonds to mould microstructure 21 and the second substrate 141as it solidifies into cast 81. Examples of thermoplastics includepoly(methyl methacrylate) (PMMA) (known by trade names such as Lucite®,Perspex® and Plexiglas®) and polycarbonate. Grades suitable for use inoutdoor settings and especially automotive applications are preferred.Most preferred are soft thermoplastics with a shore A hardness from 30to 100 including grades of low density polyethylene (LDPE).

In embodiment 201 the cast 81 is continuous with respect to a cavity 111and together with the mould 21 surrounds a cavity's fluid 71. The fluidis sealed and isolated from adjacent cavities 111. In FIG. 1C the topsurface of cast 81 is shown as 81 a and the top surface of mould 21 isshown as 21 a. Cast 81 continuously seals a cavity 111 by chemicallybonding to the second substrate 141 and to the mould microstructure 21.The latter in turn is continuously sealed by chemical bonding to thefirst substrate 101. In this way in embodiments the mould microstructure21 defines a cavity's surrounding walls and the cast microstructure 81defines a cavity's fluid sealant.

The fluid laminating step (described earlier) substantially forces thefluid out of the contact area between the cast prepolymer 41 and thesecond substrate 141. Applying compression force while laminating bringsthe prepolymer 41 into intimate contact with the second substrate 141and excess prepolymer is squeezed from recess 31 and onto the top of therecess walls 21 a in a thin layer. In some embodiments the casting stepalso seals cavities by polymerizing an excess prepolymer thin layerbetween the top of the mould microstructure 21 and the second substrate141. The cured thin layer is also known as flashing. Preferably thecured thin layer has a thickness of less than 5 micron, more preferably,less than 3 microns, and most preferably, less than 2 microns. In someembodiments excess polymer from the cast microstructure extends beyondthe mould top surface 21 a into the cavity side of the recess walls.

Advantageously in embodiment 201 the fluid 70 has little exposure to thecast's prepolymer 41 as the latter is contained in recess 31. Laminationsqueezes the fluid 70 from the contact area of the prepolymer 41 withthe second substrate 141 affording little exposure of prepolymer 41 tothe fluid during the lamination step and substantially isolatingprepolymer 41 (a high viscosity fluid) from the optical fluid 71. Thelamination step is a non-permanent sealing of prepolymer 41 betweenrecess 31 and top substrate 141 Immediately after lamination thepolymerization step cures the prepolymer making the seal permanent (i.e.by forming cast 81). During polymerization the prepolymer bulk in therecess 31 has no contact with the fluid 70. The only possible contact iswith any excess prepolymer squeezed into the cavity during lamination.By selecting and controlling the volume of prepolymer 41 printed intorecess 31 excess prepolymer can be minimized or avoided as desired.

In some embodiments the juxtaposed parallel spaced apart (from the firstsubstrate) major surface of the second substrate 141 has a polymerinsulating and/or adhesive layer over its electrode layer 60 (not shownin FIG. 1C). In some embodiments the polymer layer is polymerized at thesame time as polymerizing the prepolymer 41 of cast 41. In this way theadhesive layer enhances the peel adhesion of the cast 41 to the secondsubstrate.

In device 201 the cell gap 121 is less than or equal to the wall height1021 of mould microstructure 21 (see FIG. 1A). Advantageously in someembodiments fluid 71 is under suction within cavities 111 because thewalls 21 b are under compression or load from the fluid laminating stepresulting in a reduced wall height within devices corresponding to thecell gap 121. In this document a fluid under suction refers to a fluidthat is at a lower pressure to the atmospheric pressure of surroundings.In embodiment 201 a wall height 121 is less than the height outside thedevice 1021, and preferably the wall height within the device is lessthan or equal to 0.99 times the height outside the device.

FIG. 2A shows the first substrate 102 of embodiment 202. The latter isshown in FIG. 2B. Embodiment 202 is similar to embodiment 201 describedearlier. The walls (or sides) of microstructure 22 that define recess 32are indicated by 22 a and shown in magnified view 1002. The wall heightis 1022 and its width is 1052. The width of recess 32 is indicated by1042 and its height by 1032. The recess 32 has outward sloping curved(or rounded) walls as indicated by 22 a in magnified view 1002. In FIG.2A the walls 22 a of the recess 32 narrow to an edge on the sideopposite the first substrate.

In FIG. 2B the cavities are 112 and are filled with fluid 72. The cellgap is 122. The walls 22 a of the recess 32 narrow to an edge oncontacting (or approaching, or adjacent) the second substrate 142.Consequently the top surface 82 a of cast 82 overlaps substantially allof the top surface 22 a of mould 22 as shown in FIG. 2B. Advantageously,when cast 82 has a colorant and mould 22 is transparent, a viewerperceives both microstructures as being coloured when viewing a viewingface of the embodiment.

In some embodiments the material of the mould and cast microstructuresis the same and in others there are differences. In preferredembodiments the mould part is optically transparent and the cast partobscures light and includes one or more of a colorant (pigment or dye),a filler material, or a light scattering material. Preferably, the colorof the colorant is selected to match the colour or tint of one or moreswitchable light states of the switchable light modulator device. Forexample, an embodiment that has black, clear, and intermediate tintedstates has optically clear mould microstructures comprising polymer andblack cast microstructures comprising carbon black loaded polymer. Inanother example an embodiment that has white, clear, and intermediatetinted states has optically clear mould microstructures comprisingpolymer and white cast microstructures comprising titanium dioxideloaded polymer. In some embodiments having a coloured extreme lightstate the cast microstructures are black to minimise haze and colourperception in the clear light state.

A particular advantage of keeping the mould part optically transparentis that when it is formed by an embossing process that relies on rapidultra-violet (UV) initiated polymerization then absorption of the UV isminimized By contrast, if the mould has light absorbing material thenpolymerisation of deep wall sections (e.g., 20 microns or more) would atleast be slowed and most likely would not be possible. In a roll-to-rollprocess equipped with an embossing drum the mould precursor will haveseconds to cure before releasing/peeling from the drum surface. Withsuch a manufacturing process for the mould part it is important to usean optically transparent precursor. Advantageously in embodiments thecast part is cast in place in the device and so cast parts with lightabsorbing material can be thermally cured over a suitably long timeperiod.

FIG. 3A shows the first substrate 103 of embodiment 203. The latter isshown in FIG. 3B. Embodiment 203 is similar to embodiments 201 and 202described earlier. The walls (or sides) of microstructure 23 that definerecess 33 are indicated by 23 a and shown in magnified view 1003. Thecavity walls are indicated by 23 b, their height is 1023, and width,1053. The recess 33 has outward sloping walls as indicated by 23 a inmagnified view 1003. In FIG. 3A the walls 23 a of the recess 33 narrowto a ledge on the side opposite the first substrate. The width of recess33 is indicated by 1043 within the recess and as 1063 between the ledgeareas where the recess is at its widest. The height (or depth) of therecess is indicated by 1033. The recess 33 has a “V” shaped crosssection. In some embodiments moulds have differences in the shape oftheir recesses including variation in the shape, or depth or width ofthe recesses.

In FIG. 3B, the cavities are 113 and are filled with a liquid crystalfluid 73. The cell gap is 123. The walls 23 a of the recess 33 narrow toa ledge on contacting (or approaching, or adjacent) the second substrate143. Consequently the top surface 83 a of cast 83 overlaps substantiallyall of the top surface 23 a of mould 23 as shown in FIG. 3B. The secondsubstrate 143 comprises substrate 142 (shown in FIG. 2B) and a liquidcrystal alignment layer 193. Advantageously, the alignment layer 193 canbe coated onto the electrode surface of substrate 142 before the liquidcrystal fluid is laminated. Subsequently, the cavities 113 are sealed bycuring the cast 83. Sealing does not interfere with the alignment layer193 where it is in contact with the liquid crystal 73.

In FIGS. 3A and 3B the wall feature 23 b of the mould microstructure 23has bracing features 23 c. The latter are included to provide additionalstrength to the walls. This is beneficial when releasing the mouldmicrostructure for the embossing tool (described earlier) andsubsequently when the embodiment is laminated between glass panes. Thewidth of bracing feature 23 c is shown as 1073 and its height as 1083 inmagnified view 1003. In some embodiments the height is the same as thewall height 1023 and preferably in such devices the bracing feature hasa recess and the recess is joined to the cavity wall recess. In this waythe bracing feature also has an associated cast part and adds to thepeel adhesion of the device (the peel adhesion refers to the adhesionbetween the first and second substrates).

Device 204 is shown in FIG. 4 and shares many elements with device 201(shown in FIG. 1C). The common elements are indicated with the samenumbers in both figures. The second substrate 144 of device 204 isdifferent to the second substrate 141 of device 201. In FIG. 4 thesecond substrate 144 is shown fixed to optional release liner 154. Asimplied by its name, release liner 154 is a sacrificial layer that isintended to be removed when the device is in use (or before in amanufacturing step). The second substrate 144 is continuous and itsthickness (or dimension orthogonal to its major faces) is between 0.5microns and 50 microns, preferably between 1 micron and 35 microns, andmost preferably between 1.25 microns and 25 microns.

In some embodiments this thin sheet (i.e. second substrate 144) can be athin solid polymer and function as one or more of: a covering layer forcavities, an insulating layer, a barrier layer, or a hard coat. In someembodiments the second substrate 144 is optically clear, in otherembodiments it has colorant, and in yet other embodiments it reflectssunlight. A distinguishing feature of the second substrate 144 (device204) w.r.t. second substrate 141 (device 201) is the lack of anelectrode layer 60 on the former.

In embodiment 204 cast microstructure 84 is analogous to cast 81 inembodiment 201. Cast 84 is strongly bonded to the second substrate 144and to the mould microstructure 21. Because the cast microstructure 84is cast in the volume between the mould's recess 31 and the inner faceof the second substrate 144, it is surrounded and enclosed by both, andderived and defined by them both. The cast 84 replicates both interfacesurfaces and is a 3D imprint of their surfaces and the volume therebetween.

In FIG. 4 , embodiment 204's fluid layer is indicated as 74 and itoccupies part of the volume defined between the electrode 60 ofsubstrate 101 and the inner face (or interface) of the second substrate144. The cell gap 124 corresponds to the orthogonal distance between therespective interfaces of fluid layer 74 with the first and secondsubstrates. Fluid 74 is divided into discrete fluid volumes by the walls21 b that in turn are part of mould microstructure 21 with each fluidvolume being defined by a cavity 114. In embodiment 204 the cast 84 iscontinuous with respect to a cavity 114 and together with the mould 21surrounds a cavity's fluid 74, sealing and isolating the fluid fromadjacent cavities 114.

In some embodiments of device 204 an electrode 60 on the first substrate101 is patterned into segments and in use the fluid 74 is subjected tothe influence of an electrical field by applying different voltagepolarities and/or levels to adjacent segments. In such a device thesecond substrate may not have an electrode layer associated with it(i.e. the device forms light states with a single electrode layer andcan be said to use in-plane switching).

Embodiment 205 is shown in FIG. 5 and includes embodiment 204 asindicated (with release liner 154 removed) fixed to an active matrixbackplane 165. The fixing can be by any suitable means including byadhesive (not shown in FIG. 5 ). If an adhesive/polymer layer is usedthen preferably its thickness is kept to the minimum necessary (i.e.from 0.5 micron to 15 microns) to uniformly fix device 204 to backplane165 and achieve adequate peel adhesion between the parts. The activematrix backplane 165 has electrodes patterned to form pixels andtogether with active matrix transistors allow device 205 to operateembodiment 204 as a matrix of pixel areas from which arbitrary imagescan be displayed. Examples of products (205) include ebook readers andelectronic shelf labels.

In some embodiments, a switchable light modulator device includes one ofthe following types, or hybrid versions thereof: an electrophoreticdevice, a liquid crystal device, an electro-wetting device, anelectrokinetic device, an electrochromic device incorporating anelectrolytic fluid/gel, a thermochromic device, or a photochromicdevice. Advantageously in some embodiments the fluid layer has contactwith part of the juxtaposed parallel spaced apart major surfaces of thesubstrates including a substrate surface comprising: an electrode layer(60), an inorganic dielectric layer, an organic dielectric layer, analignment layer (193), an electrochromic layer, an ion storage layer, oran active matrix layer. In electrochromic embodiments the fluid is anelectrolytic gel and has contact with an electrochromic layer thatoverlays an electrode on one substrate and an ion storage layer thatoverlays the other electrode on the other substrate. An example of anelectrochromic device is described in Gentex's U.S. Pat. No. 6,934,067.In a hybrid electrochromic/photochromic embodiment the switchablematerial is a liquid or gel. The switchable liquid or gel is describedin Switch Material's U.S. Pat. No. 8,837,032. In a liquid crystal devicethe fluid is preferably a chiral nematic liquid crystal and a suitabledevice is described by the applicant in British Patent Application No.1416385.1 titled “A Chiral Nematic Liquid Crystal Light Shutter”. Anelectrokinetic device is a hybrid of an electrophoretic device andcomprises an ink that includes charged particles suspended in a fluid;see for example Crown Electrokinetics US 2019/0256625. In anelectrowetting embodiment the fluid layer can comprise fluids describedin Sun Chemical Corp.'s U.S. Pat. No. 8,854,714.

To enhance peel adhesion in some embodiments isolated mould and castparts can be located within a cavity. For example, a cavity can have acentrally located post with a recess (the mould microstructure) and bebonded to the opposing substrate through a cast microstructure cured inits recess. This provides peel adhesion within a cavity that supplementsthe peel adhesion provided by the cavity walls. The centrally locatedmicrostructures also act as additional spacers in the fluid layer makinga device more resistant to externally applied point pressure.

In some embodiments a peripheral edge seal about the viewing area usesmould and cast polymer parts. The mould part of the edge seal isreplicated onto its substrate in a moulding or embossing step at thesame time as the mould microstructures are replicated. Devices made withsuch a peripheral edge seal are suited to the volume production ofidentical devices such as automotive sunroofs or visors. The device canbe produced as a repeated device on a continuous roll of film and thenstamp cut or laser cut from the roll of film.

In some embodiments the added peel adhesion of the peripheral edge sealis well suited to more extreme conditions such as when the device'sedges are exposed. For example, a smart window embodiment can be bondedto a glass pane on one side only leaving its other substrate and edgearea exposed.

The substrates (101, 102, 103, 141, 142, 143) can be any suitabletransparent sheet material such as polymer or glass and can be flexibleor rigid. Flexible substrates include a polymer such as PET (i.e.polyethylene terephthalate), PEN (i.e. polyethylene napthalate), PES(i.e. polyether sulfone), PC (i.e. polycarbonate), PI (i.e. polyimide),or FRP (i.e. fiber reinforced plastic), or flexible glass (e.g., 50micron or 100 micron glass from Nippon Electric Glass Co. Ltd.). A rigidsubstrate can use float glass, or heat treated float glass, or polishedglass, or tinted/colored glass, or heat absorbing/reflecting glass, oran active matrix glass.

Electrodes (60) can be any suitable transparent conductor. For example,ITO (i.e. indium tin oxide), carbon nanotubes, silver nanowires, or aconductive polymer such as PEDOT (i.e. poly(ethylenedioxythiophene). Atop electrode can be one type such as ITO and a bottom electrode anothertype such as PEDOT. PEDOT coated PET substrates are available from Kodak(US), and ITO coated PET substrates are available from Sheldahl (US).

In flexible embodiments the microstructures and the substrates havesufficient flexibility to allow the device to conform to the curvatureof a cylinder of radius 300 mm, and preferably, radius 100 mm, and mostpreferably, radius 50 mm

As described earlier for some embodiments, the polymer used in a castmicrostructure is cured by photo or thermal means and covalently bondsto its surrounding mould microstructure and the inner surface of itsboundary substrate. Preferably the cast's prepolymer is not soluble inthe fluid of the fluid layer and has a majority by weight of highmolecular weight components and a high viscosity. In some embodimentsthe mould and cast microstructures are at least as flexible as adevice's substrates.

A suitable flexible (or deformable) polymer for use preferably in themould (21, 22, 23) and cast (81, 82, 83) microstructures of embodimentsincludes thermosetting polymers and more especially, elastomeric solidpolymer. The elastomer is characterized by a glass transitiontemperature (i.e. Tg) less than 20 degrees Celsius (i.e. 293 K) andpossessing crosslinks. In some embodiments Tg is less than the minimumoperating temperature required for an application. In some embodimentsthe rigidity of a microstructure's elastomer polymer can be selectedusing the level of crosslinking. In some embodiments an elastomer can befilled with dispersed hard material (i.e. filler) to increase itsrigidity, tear strength and durability under loading. Examples of fillermaterial include precipitated silica, fumed silica, ground quartz, blackpigment nanoparticles, carbon fibers or nanoparticles, or ceramic fibersor nanoparticles. In embodiments the elastic modulus of the solidpolymer is selected to provide suitable elastic deforming of the mouldand cast microstructures, and the modulus lies in the range 2MPa to 200MPa, and more preferably, 3 MPa to 100 MPa. In embodiments the tearstrength of the solid polymer used in the microstructures is selected tolie in the range 7.5 kN/m to 75 kN/m at 20 degrees Celsius, and morepreferably, 9 kN/m to 50 kN/m. The minimum tear strength at the maximumoperating temperature (e.g., 90 degrees Celsius) is selected to be ≥7.5kN/m. In embodiments the linear thermal expansion coefficients of thepolymers used in the mould and cast microstructures are matched.

In preferred embodiments the elastomer for one or both microstructureparts (i.e. the mould and cast parts) is polyurethane (i.e. containspolyurethane linkages). Preferred polyurethanes haveacrylate/methacrylate groups that are cured to form crosslinks. In someembodiments the polymer precursor formulation has di-functionalpolyurethane chains in solution with mono-functional monomers. Both ofthese components can be fluorinated to improve chemical resistance toswelling by an embodiment's fluid (71, 72, 73, 74). Commerciallyavailable examples of optical grade prepolymer suitable for use as theelastomeric polymer in embodiments includes the following from NorlandProducts (www.norlandprod.com): NOA78, NOA75, NOA68, NOA68T, andfluorinated grades NOA142, NOA139, NOA, 138, and NOA13825.

To minimize haze some embodiments match the refractive indices of themould and cast microstructures to the fluid, preferably to within 0.02of each other, more preferably, 0.005, and most preferably, 0.002. Otherembodiments include a colorant in the polymer of the castmicrostructures to absorb and/or reflect light. Preferably solarpigments that reflect the sunlight infra-red spectrum are used for thecolorant. Preferably the colorant is black to avoid light scattering(and consequently haze). A black colorant in the solid polymer of thecast microstructures allows mismatched refractive indices for the fluidand the black solid polymer. Furthermore, embodiments that use blackcolorant in the solid polymer of the cast microstructures can usepolymer that is not optically transparent. For example, as describedearlier the solid polymer can incorporate dispersed, hard fillermaterial. In another example the polymer can have a semi-crystallinestructure.

To provide in-plane (i.e. within the electro-optical layer) switching insome embodiments the polymer of the cast microstructures is conductiveand the cast microstructures also function as cast electrodes within thedevice.

Next, moulding techniques are described to make the mouldmicrostructures within embodiments. The moulding techniques can also bedescribed as replication techniques. These and other suitablereplication techniques are described in the Vlyte Innovations' EP3396446titled “An Electrophoretic Device Having a Transparent Light State”.

In a moulding technique a hard or soft tool surface is used as anegative mould master and in moulding steps the inverse of the threedimensional (3D) shape of the master's surface is transferred to (i.e.replicated) a substrate to form the mould microstructures. An example ofa hard tool surface is electroformed nickel and its surface is suitablefor making up to 100,000 replicas onto a substrate. An example of a softtool surface is cross-linked polydimethylsiloxane and it can make up to1,000 replicas. The moulding steps comprise coating the master's surfacewith a prepolymer and laminating the substrate (optionally the coatingis done as part of laminating), curing the coating to inverselyreplicate the shape of the master's surface in polymer bonded to thesubstrate, and peeling from the master leaving the replicatedmicrostructure on the substrate.

The mould microstructures of a device can be repeated (by replication)continuously on a roll of film in a roll-to-roll process. In this case,the surface of a drum is the hard tool. Alternatively, a continuous rollof film can be cut into sheets corresponding to a device, and then themould microstructures replicated on each substrate in a sheet process.In this case, an electroformed sheet is a suitable hard tool or P(DMS)on PET is a suitable soft tool.

A hard, negative, mould master can be made from a polymer template byelectroforming nickel onto the template's surface and therebytransferring the polymer template's shape to the surface of a hard mouldmaster. The polymer template's surface is directly formed by opticallywriting a microstructure into a photosensitive polymer known as aphotoresist and developing the resist. The direct writing of thetemplate's surface in a photosensitive polymer includes the technologiesdescribed as direct-write lithography, single-point laser writing, laserinterferometry, and electron-beam lithography. Any suitable photoresistcan be used including the SU8 series available from www.microchem.com.Directly writing the microstructure exposes the photosensitive polymerand the exposed structure is developed in solution in a separate step.Preferably, a computer controlled system uses a laser beam or electronbeam (e-beam) to expose the photosensitive polymer and form the mouldmicrostructures with wall features and recess features. Prior toelectroforming the negative mould master on the surface of the polymertemplate, the template is made more compatible (with electroforming) bydepositing a thin (<250 nm) metallic or ceramic conformal coating (orcoatings) onto its polymer surface.

In other techniques, the three-dimensional surface in a hard master(e.g., stainless steel, copper, electroformed nickel, silicon, fusedsilica, or calcium fluoride) is directly formed by material removal. Thehard surface can be formed by mechanical milling (e.g., Single PointDiamond Turning), chemical etching, ion-beam milling, reactive-ionetching, or laser ablation to directly form (or write) the replicatingsurface. Typically, the inverse image (i.e. the negative) is directlyformed in a small area called a tile and metal foil copies of this area(called shims) used to cover the tool surface, such as the surface of anembossing drum. It will be apparent to those skilled in the art thatnumerous changes and modifications can be made in the specificembodiments of the present invention described above without departingfrom the scope of the invention. Accordingly, the whole of the foregoingdescription is to be construed in an illustrative and not in alimitative sense.

All of the foregoing published patents, publications, and pendingapplications are incorporated by reference herein in their entireties.

The invention claimed is:
 1. A switchable light modulator device (201,202, 203, 204, 205) having a first substrate (101, 102, 103) and asecond substrate (141, 142, 143, 144) with opposite major surfacesspaced apart by polymer structures that create a plurality of cavities(111, 112, 113, 114), said cavities sealing a fluid comprisingelectrophoretic particles, wherein the polymer structures includerecesses (31, 32, 33) that are filled with conducive cast parts (81, 82,83, 84), the conductive cast parts being bonded to both the secondsubstrate and a surface of said recess, the conductive cast parts beingenclosed by said surface of said recess and said second substrate. 2.The switchable light modulator device of claim 1, wherein the conductivecast parts comprise a polymer and a conductive filler.
 3. The switchablelight modulator device of claim 1, wherein the polymer structures areoptically transparent and the conductive cast parts (81, 82, 83, 84)obscures light and include a colorant or a light scattering material. 4.The switchable light modulator device of claim 1, wherein the recesses(31, 32, 33) vary in depth (1031, 1032) or width (1041, 1042).
 5. Theswitchable light modulator device of claim 1, wherein the polymerstructures additionally include bracing features (23 c).
 6. Theswitchable light modulator device of claim 1, wherein the firstsubstrate (101, 102, 103) or the second substrate (141, 142, 143, 144)comprises a flexible transparent material (90).
 7. The switchable lightmodulator device of claim 6, wherein the first substrate (101, 102, 103)or the second substrate (141, 142, 143, 144) additionally comprises atransparent electrode layer (60).
 8. The switchable light modulatordevice of claim 1, wherein the switchable light modulator device (201,202, 203, 204, 205) has a first state that strongly attenuates light,and a second state that is substantially transparent to visible light.9. A display, a window, a mirror, a sun shade, or a sign including aswitchable light modulator device (201, 202, 203, 204, 205) of claim 1.